Reduction of artifacts in fluorescence correlation spectroscopy due to sample adsorption on optical glass surfaces

Spectroscopic and solution properties characterization of quaternary ammonium ion containing polycations complexed with fluorescent rhodamine sulfonic acid dyes

Based on the growing range of applications for polycations in research and commercial materials, a continuing need exists to advance the fundamental knowledge and understanding of this class of materials. Spectroscopic and solution properties characterizations of noncovalently labeled, fluorescent Alexa Fluor® dye complexes of two commercial polycations, poly(2-(trimethylamino) ethyl methacrylate) monocation and poly[bis[2-chloroethyl] ether-alt-1,3-bis[3-(dimethylamino) propyl] urea] dication are reported to help address this need. A variety of fluorescence spectroscopic methods are used with a special emphasis on fluorescence correlation spectroscopy (FCS) which is applied to characterize the Stokes radius (RS) and equilibrium dissociation constants (Kd) of dye-polycation complexes at nanomolar dye concentrations. Resulting RS values indicate dye binding to individual polycation chains. Measured Kd values in the sub-micromolar range are consistent with strong dye binding. Increasing solution ionic strength with sodium chloride addition inhibits dye binding and decreases the RS of dye-polycation complexes due to size collapse of polycation chains. The complexes differ in their solution stability to ionic strength changes suggesting that both electrostatic and hydrophobic binding interactions influence dye binding. This study establishes the viability of noncovalent dye-polycation complexation in concert with FCS characterization as a general approach for investigating the properties of quaternary ammonium ion containing polycations in aqueous solution.

Spectroscopic characterization of the interactions between poly(2-(trimethylamino)ethyl methacrylate) chloride and the xanthene dyes, 2', 7'-difluorofluorescein and 2, 4, 5, 7-tetraiodofluorescein

Intermolecular interactions in buffered aqueous solution between the polycation, poly(2-(trimethylamino)ethyl methacrylate) chloride (pTMAEMC) and two anionic xanthene dyes, 2', 7'-difluorofluorescein (Oregon Green 488) and 2, 4, 5, 7-tetraiodofluorescein (Erythrosin B), are characterized using multiple optical spectroscopic methods. Visible absorption spectroscopy indicates the formation of ground-state pTMAEMC-dye complexes. Benesi-Hildebrand binding isotherm analysis of visible absorption spectra for pTMAEMC-dye mixtures quantifies the strength of binding interactions producing the complexes. For both Oregon Green 488 (OG) and Erythrosin B (EB) in mixtures with pTMAEMC, the concentration of the solution's sodium acetate buffer at a fixed pH alters the binding constants, K_b, suggesting that ionic strength plays a key role in determining the binding affinity of pTMAEMC for the dyes. Comparison of K_b, for the dyes indicates stronger binding of EB under all solution conditions. Steady-state fluorescence emission spectroscopy, fluorescence quenching, excited-state fluorescence lifetime measurements and fluorescence correlation spectroscopy provide complementary data for the interactions between pTMAEMC and the dyes. Mixtures of pTMAEMC with the dyes produce fluorescence enhancements and fluorescence quenching which exhibit a dependence on the buffer concentration used in the mixture. Excited-state lifetime analysis indicates that OG interacts with pTMAEMC through ground-state interactions while EB exhibits both ground-state and excited-state interactions with pTMAEMC. The spectroscopic measurements suggest that a polyelectrolyte effect for pTMAEMC due to ionic strength variation produced by the buffer concentration affects the dye binding profile of the polycation. This conclusion is supported by fluorescence correlation spectroscopy (FCS) analyses of the hydrodynamic diameter changes in pTMAEMC-OG binding in low buffer concentration (low ionic strength) solution. FCS analyses of pTMAEMC-OG mixtures also reveal diversity in the complexes formed in low ionic strength solution suggesting that other xanthene dyes will exhibit similar binding behaviors in mixtures with pTMAEMC as a function of solution ionic strength.

Supported Solid Lipid Bilayers as a Platform for Single-Molecule Force Measurements

Biocompatible surfaces are important for basic and applied research in life science with experiments ranging from the organismal to the single-molecule level. For the latter, examples include the translocation of kinesin motor proteins along microtubule cytoskeletal filaments or the study of DNA-protein interactions. Such experiments often employ single-molecule fluorescence or force microscopy. In particular for force measurements, a key requirement is to prevent nonspecific interactions of biomolecules and force probes with the surface, while providing specific attachments that can sustain loads. Common approaches to reduce nonspecific interactions include supported lipid bilayers or PEGylated surfaces. However, fluid lipid bilayers do not support loads and PEGylation may require harsh chemical surface treatments and have limited reproducibility. Here, we developed and applied a supported solid lipid bilayer (SSLB) as a platform for specific, load bearing attachments with minimal nonspecific interactions. Apart from single-molecule fluorescence measurements, anchoring molecules to lipids in the solid phase enabled us to perform force measurements of molecular motors and overstretch DNA. Furthermore, using a heating laser, we could switch the SSLB to its fluid state allowing for manipulation of anchoring points. The assay had little nonspecific interactions, was robust, reproducible, and time-efficient, and required less hazardous and toxic chemicals for preparation. In the long term, we expect that SSLBs can be widely employed for single-molecule fluorescence microscopy, force spectroscopy, and cellular assays in mechanobiology.

Variable surface transport modalities on functionalized nylon films revealed with single molecule spectroscopy

Functionalization of separation membranes with ion-exchange ligands allows control of the surface mobility of protein molecules facilitating optimized membrane design.

A high-throughput platform for real-time analysis of membrane fission reactions reveals dynamin function

Dynamin, the paradigmatic membrane fission catalyst, assembles as helical scaffolds that hydrolyse GTP to sever the tubular necks of clathrin-coated pits. Using a facile assay system of supported membrane tubes (SMrT) engineered to mimic the dimensions of necks of clathrin-coated pits, we monitor the dynamics of a dynamin-catalysed tube-severing reaction in real time using fluorescence microscopy. We find that GTP hydrolysis by an intact helical scaffold causes progressive constriction of the underlying membrane tube. On reaching a critical dimension of 7.3 nm in radius, the tube undergoes scission and concomitant splitting of the scaffold. In a constant GTP turnover scenario, scaffold assembly and GTP hydrolysis-induced tube constriction are kinetically inseparable events leading to tube-severing reactions occurring at timescales similar to the characteristic fission times seen in vivo. We anticipate SMrT templates to allow dynamic fluorescence-based detection of conformational changes occurring in self-assembling proteins that remodel membranes.

Spatial Control of Epsin-induced Clathrin Assembly by Membrane Curvature

Epsins belong to the family of highly conserved clathrin-associated sorting proteins that are indispensable for clathrin-mediated endocytosis, but their precise functions remain unclear. We have developed an assay system of budded supported membrane tubes displaying planar and highly curved membrane surfaces to analyze intrinsic membrane curvature preference shown by clathrin-associated sorting proteins. Using real-time fluorescence microscopy, we find that epsin preferentially partitions to and assembles clathrin on highly curved membrane surfaces. Sorting of epsin to regions of high curvature strictly depends on binding to phosphatidylinositol 4,5-bisphosphate. Fluorescently labeled clathrins rapidly assemble as foci, which in turn cluster epsin, while maintaining tube integrity. Clathrin foci grow in intensity with a typical time constant of ∼75 s, similar to the time scales for coated pit formation seen in cells. Epsin therefore effectively senses membrane curvature to spatially control clathrin assembly. Our results highlight the potential role of membrane curvature in orchestrating the myriad molecular interactions necessary for the success of clathrin-mediated membrane budding.